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Metal-organic frameworks (MOFs) have emerged as interesting candidates for intracellular carrier-based delivery. These hybrid materials are constituted of metal clusters linked together by organic ligands. The possibility to tune their physical and chemical properties both in the bulk and at the surface allows for the design of biocompatible delivery systems with high loading capacities and targeting abilities, combining the benefits of both organic and inorganic materials. The following dissertation focuses on developing and evaluating MOFs as intracellular delivery systems. In the first instance, a zirconium-based MOF, UiO-66, was synthesised and utilised as an intracellular delivery vector for trehalose, a disaccharide with cryoprotective properties when present in the cytosol. This MOF demonstrated very high trehalose weight loadings compared to other trehalose delivery systems (up to ca. 50 wt %), release of the sugar from the framework over 5 h, and appropriate biocompatibility. To assess the delivery system's impact on cryopreservation, the viability of cells cryoprotected with trehalose-loaded UiO-66 was tested at 0 h, 24 h, and 48 h post-thaw, and showed no improvement compared to cells frozen with free trehalose or growth media alone. The absence of cryoprotective effect was hypothesised to be due to endosomal entrapment of the delivery system after cellular uptake through endocytosis. The final fate of particles taken up by cells depends on the endocytosis pathways they go through. In order to confirm the hypothesis of MOF endosomal entrapment, the endocytosis of MOF particles was studied. In particular, the effects of surface chemistry of Zr-based MOFs on their endocytosis mechanisms were investigated. It was found that MOF surface chemistry had an important effect on cellular uptake behaviour, whereas particle size played a less important role. In particular, Zr-based MOFs synthesised using naphthalene-2,6-dicarboxylic acid and 4,4'-biphenyldicarboxylic acid as organic ligands, and UiO 66 particles surface-decorated with folic acid and PEG, promoted entry through the caveolin-pathway. This allowed the particles to potentially avoid endosomal entrapment and reach the cytosol, enhancing their therapeutic activity when loaded with drugs. Equipped with an understanding of the cellular uptake of MOF particles, a range of mitochondrially-targeted UiO-66 particles capable of bypassing endosomal entrapment was prepared and tested. The UiO-66 particles were loaded with dichloracetic acid (DCA), a small chemotherapeutic drug molecule that acts on mitochondria, and surface-functionalised with triphenylphosphonium, a known mitochondrial targeting agent. The system demonstrated a dramatic increase in efficacy, allowing a reduction in DCA effective dose of ca. 100-fold compared to the free drug, and ca. 10-fold compared to non-targeted, DCA-loaded UiO-66. Confocal microscopy revealed a distribution of the targeted nanoparticles around mitochondria. Super-resolution microscopy of cells treated with the system revealed important mitochondrial morphology changes associated with cell death as soon as 30 minutes after incubation. A whole transcriptome analysis of cells treated with the system indicated widespread changes in gene expression compared to both untreated cells and to cells treated with non-targeted, DCA-loaded UiO-66. In summary, these studies demonstrated the advantages of MOFs as targeted intracellular delivery vectors. The ease with which their physicochemical properties can be tuned allows for the design of delivery systems able to bypass the critical drug delivery bottlenecks of endosomal entrapment and non-specific delivery.